This info was once posted on another forum, courtesy of ZeGerman: E36 Cooling System 101: What you need to know

Why does the cooling system need to be replaced every 75-100k miles?

The E36 BMW is by many accounts a very reliable car. They can last for hundreds of thousands of miles with little more than routine maintenance if a few key steps are followed. By a wide margin, the biggest weakness of the E36 BMW in terms of reliability is the cooling system. As these cars age, the plastic used in the cooling system components becomes very brittle, and by around 75-100k miles, they are at high risk of a catastrophic failure, which can produce the following outcomes:

- Cracked/sheered upper or lower radiator hose flange
- Cracked radiator tank(s)
- Radiator leaks at tank/core seams
- Leaking radiator cap
- Cracked/exploding expansion tank
- Cracked bleeder screw
- Cracked/sheered thermostat housing
- Ruptured upper/lower radiator hoses
- Water pump impeller failure
- Water pump bearing failure
- Mechanical/clutch fan failure/explosion

Failure of any of the above items can and will cause the engine to overheat. When most people acquire a new-to-them E36, they will inspect the cooling system visually, and if there are no leaks and the engine is not overheating, they assume that everything is fine and will continue driving the car as-is. The reason why this is a risky scenario is because the failure-prone cooling system components will look and function perfectly until they fail suddenly and without warning, thus leading to an overheating incident. In other words, visual inspection is not a sufficient means of determining the lifespan of cooling system components. Something can look brand new today, and then break tomorrow.

What are the consequences of a cooling system failure?

As mentioned earlier, the primary motivation for replacing all cooling system components every 75-100k miles is to avoid overheating the engine in the first place - this is preventive maintenance. The reason why avoiding an overheating scenario is so important with these cars is due to the fact that they utilize aluminum cylinder heads, which are extremely susceptible to warping and cracking from overheating. Warped heads and blown head gaskets are very common occurrences with the E36, but fortunately, this problem can largely be avoided if you overhaul the cooling system before something breaks. Generally speaking, the cost to repair a blown head gasket and/or warped (or cracked) cylinder head is in the ballpark of $1200 (sometimes more, sometimes less), and is not a job that is within the skill set of most of DIY'ers. On the other hand, the cost of a comprehensive cooling system overhaul is in the ballpark of $350-450 (parts only), and is within the skill set of most average DIY'ers. So, it's easier and less expensive to replace the cooling system before you have a problem on your hands.

Many people take the risk and figure that they will simply drive the car until something breaks, and when it does break, they assume they will notice that the temperature gauge has started to rise and will be able to shut the engine off before it has truly overheated. There are a few reasons why this is not a wise strategy:

- When driving, one spends most of the time looking through the windshield at the road ahead (thankfully), rather than constantly looking at the instrument cluster. Because of this, you may very likely not notice that the temp gauge has risen until it is too late and damage has occurred. In most cases, people won't notice until the temp gauge is buried deep in the red and the warning light illuminates, at which point your engine has been thoroughly cooked.

- The temp gauge is not accurate, or in more specific terms, the temp gauge has a built-in buffer/delay. This means that in some cases (such as in the event of an overheating), the actual coolant temperature is considerably higher than what the temp gauge is indicating. Many people figure that as long as the temp needle does not reach the red zone, they are safe. This is not true, because by the time the temp gauge has risen beyond the normal 12 o'clock position, the actual coolant temp can already be dangerously high, despite the fact that the needle hasn't yet reached the red zone. This is due to the buffer/delay.

So, now that we have established how the cooling system fails (brittle plastic), and why it is so important to avoid overheating the engine (high risk of blown head gasket and/or warped head), we are brought to our next topic of discussion:

Which cooling system components need to be replaced?

The general rule here is that pretty much everything needs to be replaced, and as mentioned at the beginning, they should be replaced every 75-100k miles or right now if you do not have proof that they have been replaced recently (this second point is of particular importance to individuals with newly acquired E36 BMWs). If the previous owner of your car did not provide you with receipts of a recent cooling system overhaul, consider your cooling system to be at-risk and in need of comprehensive replacement. Here are the parts (with part numbers) which should be included in your cooling system overhaul:

- 6 Clylinder: 17111728908
- 4 Cylinder: 17111728907

Radiator cap: 17111742231

Radiator hoses:
- 6 Cylinder: 11531708499 (upper) / 11531726344 (lower)
- 4 Cylinder (1.8L M42): 11531721708 (upper) / 11531721709 (lower)
- 4 Cylinder (1.9L M44): 11531743535 (upper) / 11531247261 (lower)

Expansion tank (6-Cyl only): 17111723520

Bleeder screw (aftermarket brass available): 17111712788

Thermostat housing:
- 6 Cylinder (aftermarket aluminum available): 11531722531, Gasket: 11531740437
- 4 Cylinder (1.8L M42): 11531721966, Gasket: 11531721172
- 4 Cylinder (1.9L M44 includes thermostat): 11531743017, Gasket: 11531743179

Thermostat: 11537511083

Water pump:
- 6 Cylinder: 11517527799
- 4 Cylinder: 11510393338

- 6 Cylinder: 11521712058
- 4 Cylinder: 11521723363

- 6 Cylinder: Main: 11281437929, A/C: 11281437873
- 4 Cylinder (1.8L M42): Main: 11281247986, A/C: 11281743193
- 4 Cylinder (1.9L M44): Main: 11281437369, A/C: 11281743193

Coolant: 82141467704

A note about water pumps

BMW used no fewer than three different styles of water pump throughout the E36 production run. They all fit interchangeably (within the same engine), but differ in the material used for the impeller. From 1992 until sometime partway through 1995, BMW installed water pumps which had plastic impellers, which were notorious for premature failure due to the plastic impeller becoming brittle and sheering off inside the engine, thus resulting in no coolant circulation and subsequent overheating. They were known for failing as early as 60k miles, and sometimes much earlier than that. After BMW finally figured out what was going on with their water pumps, they implemented an updated pump sometime in late '95 to early '96 which used a metal impeller, thus eliminating the risk of impeller failure. Also during the 1996-99 period, some E36 BMWs received water pumps with plastic composite impellers. I've never personally seen a water pump with the composite impeller, but they are out there. Supposedly they are safe to use and do not suffer the same ailment as the early pumps with regular plastic impellers. Nevertheless, pumps with metal impellers now seem to be the most widely adopted configuration for the E36, so that is what I would recommend.

One might then ask the question: "Since I own a '96-99 E36, which came with a water pump with a metal impeller from the factory, why do I still need to replace the water pump every 75-100k miles?" The answer is that while the metal impeller will not pose a risk, you do still run the risk of experiencing a bearing failure, and since OEM water pumps generally run in the ballpark of $60-80 (which is very reasonable), it makes a lot more sense to replace the pump while you are in there replacing everything else simply out of precaution.

A note about aluminum thermostat housings

Since aftermarket aluminum t-stat housings are not susceptible to cracking the same way the OEM plastic housings are, many people elect to install the aluminum t-stat housing. It makes perfect sense to perform this upgrade, and BMW should have used aluminum (or at least a more durable plastic) for the t-stat housing from the factory. That said, virtually all of the aftermarket aluminum t-stat housings available are cheap sand cast units made in China with poor manufacturing consistency. While I can only speak to this based on my own personal experience and the similar experiences of a handful of acquaintances, I cannot fully recommend the use of aluminum t-stat housings. I installed one on my car, and despite using oxygen sensor safe RTV t-stat housing sealant around the gasket and properly torqueing down the housing, I could never get it to seal correctly. It was more of a seep rather than a leak, but coolant was getting out, albeit very slowly. I've discussed this topic with other E36 BMW owners, and in doing so, I learned that I am not the only one to have had this experience. Mine was purchased from Turner Motorsport. Ultimately, the way I view them is this: If you get a good example that seals correctly, you're home free and will have a more durable alternative to the OEM plastic unit. If not, then you're going to wish you had bought the OEM plastic unit. When I received my aluminum t-stat housing, it did look to be perfectly formed, but still leaked nonetheless. I've recently switched back to the OEM plastic unit, and I am now leak-free, but I will have to be diligent about replacing it at the prescribed intervals.

A note about brass bleeder screws

Buy it there are no drawbacks or caveats.

Additional information

This guide is meant to tell you why you need to periodically overhaul the cooling system, and exactly what parts are needed to perform an overhaul. There is already a lot of information about the E36 cooling system and its shortcomings available online, including HERE on our very own forum. A simple Google search of "E36 BMW cooling system DIY" will yield a wealth of information, including step-by-step tutorials, recommendations, photos, and many sad stories of people who didn't know to include a cooling system overhaul in their general maintenance schedule (or knew about the risks, but didn't heed the warnings...). Use this guide as a supplement to all the other information that is readily accessible online and you will be in good shape.